The present invention relates to a method and system for determining consistency of stock being refined by a disk refiner as well as a method and system for controlling refiner operation based on consistency.
Many products we use every day are made from fibers. Examples of just a few of these products include paper, personal hygiene products, diapers, plates, containers, and packaging. Making products from wood fibers, cloth fibers and the like, involves breaking solid matter into fibrous matter. This also involves processing the fibrous matter into individual fibers that become fibrillated or frayed so they more tightly mesh with each other to form a finished fiber product that is desirably strong, tough, and resilient.
In fiber product manufacturing, refiners are devices used to process the fibrous matter, such as wood chips, pulp, fabric, and the like, into fibers and to further fibrillate existing fibers. The fibrous matter is transported in a liquid stock slurry to each refiner using a feed screw driven by a motor. Each refiner has at least one pair of circular ridged refiner discs that face each other. During refining, fibrous matter in the stock to be refined is introduced into a gap between the discs that usually is quite small. Relative rotation between the discs during operation causes the fibrous matter to be fibrillated as the stock passes radially outwardly between the discs.
One example of a refiner that is a disc refiner is shown and disclosed in U.S. Pat. No. 5,425,508. However, many different kinds of refiners are in use today. For example, there are counterrotating refiners, double disc or twin refiners, and conical disc refiners. Conical disc refiners are often referred to in the industry as CD refiners.
Each refiner has at least one motor coupled to a rotor carrying at least one of the refiner discs. During operation, the load on this motor can vary greatly over time depending on many parameters. For example, as the mass flow rate of the stock slurry being introduced into a refiner increases, the load on the motor increases. It is also known that the load on the motor will decrease as the flow rate of dilution water is increased.
During refiner operation, a great deal of heat is produced in the refining zone between each pair of opposed refiner discs. The refining zone typically gets so hot that steam is produced, which significantly reduces the amount of liquid in the refining zone. This reduction of liquid in the refining zone leads to increased friction between opposed refiner discs, which increases the load on the motor of the refiner. When it becomes necessary to decrease this friction, water is added to the refiner. The water that is added is typically referred to as dilution water.
One problem that has yet to be adequately solved is how to control refiner operation so that the finished fiber product has certain desired characteristics that do not vary greatly over time. For example, paper producers have found it very difficult to consistently control refiner operation from one hour to the next so that a batch of paper produced has consistent quality. As a result, it is not unusual for some paper produced to be scrapped and reprocessed or sold cheaply as job lot. Either way, these variations in quality are undesirable and costly.
Another related problem is how to control refiner operation to repeatedly obtain certain desired finished fiber product characteristics in different batches run at different times, such as different batches run on different days. This problem is not trivial as it is very desirable for paper producers be able to produce different batches of paper having nearly the same characteristics, such as tear strength, tensile strength, brightness, opacity and the like.
In the past, control systems and methods have been employed that attempt to automatically control refiner operation to solve at least some of these problems. One common control system used in paper mills and fiber processing plants throughout the world is a Distributed Control System (DCS). A DCS communicates with each refiner in the mill or fiber processing plant and often communicates with other fiber product processing equipment. A DCS monitors operation of each refiner in a particular fiber product processing plant by monitoring refiner parameters that typically include the main motor power, the dilution water flow rate, the hydraulic load, the feed screw speed, the refiner case pressure, the inlet pressure, and the refiner gap. In addition to monitoring refiner operation, the DCS also automatically controls refiner operation by attempting to hold the load of the motor of each refiner at a particular setpoint. In fact, many refiners have their own motor load setpoint. When the motor load of a particular refiner rises above its setpoint, the DCS adds more dilution water to the refiner to decrease friction. When the motor load decreases below the setpoint, dilution water is reduced or stopped.
During refiner operation, pulp quality and the load on the refiner motor vary, sometimes quite dramatically, over time. Although the aforementioned DCS control method attempts to account for these variations and prevent the aforementioned problems from occurring, its control method assumes that the mass flow of fibrous matter in the stock entering the refiner is constant because the speed of the feed screw supplying the stock is constant. Unfortunately, as a result, there are times when controlling the dilution water flow rate does not decrease or increase motor load in the desired manner. This disparity leads to changes in refining intensity and pulp quality because the specific energy inputted into refining the fibrous matter is not constant. These changes are undesirable because they ultimately lead to the aforementioned problems, as well as other problems.
In the past, consistency has been measured externally of a refiner in an effort to determine how well the refiner is operating. After evaluating the consistency measurement, there are times where an operator will manually make an adjustment to the refiner in an effort to try to get the consistency closer to a desired value or range. Unfortunately, it takes a long time, often several hours or longer, before the operator will know whether his or her adjustment had the desired impact on consistency. This hit and miss approach is inexact and inefficient.
This delay is believed to be caused by at least two problems, if not more. First, it takes a long time, often several hours or possibly even days, for the refiner to reach steady state operation before an operator will know what kind of effect that the change had. Second, consistency measurements are taken outside the refiner using equipment and methods that are slow, which also delays how fast an operator can obtain feedback. In any event, because the present methods and devices for measuring consistency are slow and there is a corresponding delay in recognizing that the refiner is operating in a steady state condition, the operator is forced to wait a long time until they know with some certainty what kind of effect their change had. This means, that the refiner can operate inefficiently for hours, if not days, before the operator, using this trial and error method, finally settles on a combination of operating settings that are more to his or her liking.
Hence, while some refiner process control methods have proven beneficial in the past, they in no way have resulted in the type of control over finished fiber product parameters and the repeatability of these parameters that is desired. Thus, additional improvements in refiner process control and consistency measurement are needed.
A system for and method of determining stock consistency. The invention includes one or more sensors that sense temperature and/or pressure of stock adjacent or in the refining zone during refiner operation. In one preferred embodiment, one or more sensors in the refining zone provide real time temperature and/or pressure data from which a consistency is determined. A plurality of sensors can be used. Sensors can be distributed radially along the refining zone to provide a distribution of temperature and/or pressure data from which a consistency is determined. Using the system and method of the invention, consistency can be quickly measured in no more than five minutes and preferably is measured in real time. In one preferred embodiment and method, consistency is measured a plurality of times a second and preferably at a rate of about twenty times a second.
The consistency can then be used to control or help control refiner operation. For example, in one preferred embodiment, the dilution water to the refiner is regulated based on stock consistency. In another preferred method, the volumetric flow rate of the stock is regulated based on stock consistency. If desired, regulation of volumetric flow rate and dilution water can both be based on stock consistency. If desired, another parameter, such as refiner gap, can be regulated based on consistency.
Where refiner temperature is used in determining consistency, the refiner temperature is a temperature of stock inside the refiner or adjacent its inlet or outlet. In one preferred implementation, the refiner temperature is a temperature of stock in the refining zone. Where there is more than one sensor in the refining zone, the temperature can be provided by a particular selected sensor or calculated based on the sensor data from more than one sensor. In one preferred embodiment, temperature measurements from multiple sensors are averaged. In another embodiment, a temperature profile using data from each sensor is used.
Where refiner pressure is used in determining consistency, the pressure preferably is a pressure inside the refiner, such as a pressure in the refining zone, or a pressure inside the refiner adjacent an inlet or outlet. Where there is more than one sensor in the refining zone, the pressure can be provided by a particular selected sensor or calculated based on the sensor data from more than one sensor. In one preferred embodiment, pressure measurements from multiple sensors are averaged. In another embodiment, a pressure profile using data from each sensor is used.
In one preferred implementation of the method of determining consistency, the method uses temperature or pressure measured inside the refining zone along with other refiner parameters in determining the consistency of stock in the refining zone as a function of time and location in the refining zone. This method advantageously permits consistency of stock to be determined in real time in the refining zone.
Where volumetric stock flow or mass flow is regulated, it preferably is regulated by controlling the speed of a feed screw that provides the refiner with stock or fiber for stock. Where dilution water flow is regulated, it preferably is regulated by controlling operation of the dilution pump. Other refiner parameters can be controlling using the method of this invention.
So that the process can be controlled despite changes in refiner operation not due to regulation using the method, one preferred implementation pauses to permit refiner operation to stabilize before resuming regulation of refiner operation. For example, where an operator manually changes refiner operation, regulation is paused preferably until refiner operation stabilizes. The same is true where a refiner is also subject to control of a processing device, such as a Distributed Control System (DCS).
In one preferred embodiment, the method is implemented in the form of a controller that preferably is a PI or a PID controller. If desired, a proportional controller can be used. The controller can be a digital or analog controller and can be configured to operate with a processor such as the digital processor of a personal computer, a DCS, a programmable controller or the like.
The system includes a processor that receives data related to refiner operation. Suitable data includes data related to the process variable or variables used in regulating refiner operation. In one preferred embodiment, the processor receives data related to one or more of the following parameters: the power inputted into the refiner, the feed screw speed (or volumetric stock flow or feed rate), the temperature of the stock before it enters the refiner, the temperature of stock after it leaves the refiner, a refiner temperature, a refiner pressure, the force exerted on the refiner disks urging them together, the dilution motor power of the dilution pump, the chip washing water temperature, the dilution water temperature, the gap between the refiner disks, as well as other parameters. At least three of these parameters are used in determining consistency.
In carrying out a method of refiner control using consistency, the processor outputs at least one control signal. Each control signal can be directly provided to the refiner or a component related to the refiner, such as the feed screw or dilution water pump. If desired, each control signal can be provided to another processor, such as a DCS, that causes the DCS to regulate the desired parameter. For example, a control signal can be provided to the DCS that causes the DCS to change feed screw speed. Another control signal can be provided to the DCS that causes the dilution water flow rate to change. Another control signal can be provided to the DCS that causes the refiner gap to change.
Using a method and system of this invention, control changes can be made to the refiner at a rate of at least one every five minutes and preferably faster. For example, in one preferred embodiment, real time control of the refiner is achieved. Preferably, the system and method enables a plurality of the aforementioned setting changes to be made per second and as fast as at a rate of about twenty hertz.
In one preferred embodiment, each sensor is carried by a refiner disk or segment of the disk. In one preferred sensor disk or sensor disk segment, each sensor is imbedded in the refining surface of the disk or segment.
In a preferred sensor embodiment, the sensor has a sensing element carried by a spacer that spaces the sensing element from the material of the disk or segment in which it is imbedded. This prevents the sensor from detecting the temperature of the disc or disc segment, and, instead, when insulated, the sensor detects the temperature of the stock. One preferred spacer is made from an insulating material that preferably thermally insulates the sensing element from the thermal mass of the refiner disk material.
Other objects, features, and advantages of the present invention include one or more of the following: a method and system for determining consistency at or close to the refining zone; a method and system for determining consistency in real time in the refining zone; a method and system for determining consistency in the refining zone as a function of position in the refining zone; a method and system for using determined consistency in controlling some aspect of refiner operation; a method and system that controls refiner operation in real time based on consistency measured in real time; and a method and system for controlling refiner operation based on consistency that accommodates changes to other refiner settings; is a method and system for measuring consistency that is reliable, economical, easy to manufacture and install, repeatable, fast, rugged, and efficient; and is a method and system for controlling refiner operation based on consistency that is also reliable, economical, fast, rugged, and efficient.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating at least one preferred embodiment of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.